1. Field of the Invention
This invention relates to heat exchangers and, more particularly, to motor vehicle heat exchangers utilizing grommets in the tube-to-header joints.
2. Description of Related Art
Heat exchangers, particularly those utilized in motor vehicles, may be liquid-to-air heat exchangers, (e.g., radiators for engine coolant, air conditioning condensers and evaporators, and oil coolers) or may be air-to-air heat exchangers (e.g. charge air coolers). Liquid-to-air and air-to-air heat exchangers are typically composed of an inlet tank or manifold, an outlet tank or manifold, and a large number of tubes extending between the tanks or manifolds which carry the fluid to be cooled. Headers are normally provided on the tanks for mechanical attachment and fluid connection of the tubes. Fins attached to the tubes transfer heat between the liquid or gas inside the tubes and the ambient atmosphere outside. A mechanical framework or structure is usually included to provide structural strength to the assembly and to provide means for mounting the unit to the vehicle or other machinery on which it is used.
As shown in FIGS. 1 and 2, a typical heat exchanger core, in this case radiator core 20, is comprised of a plurality of vertical, parallel, spaced tubes 22 between which are interposed heat transfer fins. These fins may be of the flat type 24 or the serpentine type 26 in the composite core depicted in FIG. 1. Any of these fin styles may include louvers (not shown) to enhance heat transfer. The fins are typically formed of strips of aluminum, brass, copper or other thermally conductive metal or alloy. Flat fins 24 are generally made of sheet metal which has a collar formed about a hole. Tubes 22 may be inserted through the collared openings and a plurality of fins may be stacked in order to make up the fin array within the core. Serpentine fins may extend in a serpentine pattern wherein the strips are configured with a plurality of alternating bends between adjacent tubes. The root of the bend is generally secured by brazing or soldering to the tube. A strip portion between the roots extends between the tubes. In serpentine fins 26 the pattern is similar to that of a sine wave, while in serpentine fins 30 (FIG. 2) the pattern is zig-zag. The ends of tubes 22 extend beyond the fin array of core 20 to connect to the headers and tanks.
Headers 28a and 28b are at the top and bottom, respectively, of core 20 and are plates having openings therein to receive and seal the upper and lower ends of the tubes 22. Upper and lower tanks 34a and 34b, respectively, are normally welded or soldered to headers 28a and 28b respectively and contain an inlet 36 and outlet 38 for the heat exchanger. Side support rails 32 or other structure may be used to secure the tanks and headers on either side of the core and enable the completed heat exchanger to be secured within the vehicle or machinery frame.
The tubes utilized may be either round or oval, or may be oval with circular ends. Prior art methods of welding tube-to-header joints are disclosed, for example, in U.S. Pat. No. 5,407,004, the disclosure which is hereby incorporated by reference.
In use, heat from the hot liquid or air within generally causes the tubes to expand and grow in length due to thermal expansion. Since the tanks or manifolds are fixed with respect to each other by the unit framework or structure, the growth in length of tubes places high mechanical stresses on the tanks and the associated headers, particularly in the area of the joints between the tubes and headers. In addition, the pressure of the hot liquid or hot air within the heat exchanger tends to distort the tanks or manifolds and headers, creating further stresses on the tube-to-header joints. The combination of stress resulting from thermal expansion and internal pressure can result in early failure of heat exchangers. Cracks in the joints between the tubes and the headers are the most common mode of failure. Many approaches have been taken to avoid heat exchanger failures due to thermal expansion and internal pressure. Most approaches fall into one or two categories: 1) those which improve the strength of the areas prone to failure and 2) those which provide resilience in the areas prone to failure. Approaches which provide resilience have appealed to designers because they provide a solution to the stresses of thermal expansion and internal pressure with a greater economy than any approach which must provide more material to achieve an improvement in strength.
Engine cooling radiators for vehicles have sometimes been designed with resilient tube-to-header joints. Locomotive radiators have been manufactured by the assignee of the present invention for over thirty (30) years using headers of special resilient design as shown in FIG. 3. Metallic headers 28 are mechanically attached to tanks (not shown), such as by bolting, and have oversized holes or openings in them to receive oval brass tubes 22 extending from the radiator core. Fins 24 of the flat plate-type design have collars 25 fitted around the tubes. Within the openings in the header there are placed oval brass ferrules 44. These ferrules are bonded to the header by molded silicone rubber 40. The ferrules are then soldered to the core tubes extending therethrough to form a leak-free, resilient joint between the tubes and the headers. While this has been an extremely effective design under typical operating conditions for locomotives, it is expensive to produce.
In the 1970's, radiators for automobiles were produced which utilized round aluminum tubes, aluminum plate fins, aluminum headers and plastic tanks. A sheet of molded rubber provided resilient grommets at each tube hole in the header, and also provided a gasket for sealing the headers to the plastic tanks, which were attached to the headers by means of crimped tabs on the headers. The insertion of the tubes into the rubber grommets in the header holes compressed the rubber of the grommets providing a resilient sealing attachment of the tubes to the headers. However, considerable force was required to insert all the core tubes into the header holes simultaneously. This design was limited to relatively small units because of the problems of core and header distortion during assembly and because of the close tolerance which was required to accomplish the mating of the core tubes to the header with the desired amount of grommet compression.
Other radiators have also utilized rubber grommets in their tube-to-header joints. These radiators have been designed around individual finned tubes having round ends and oval cross-sections which are finned along most of their length. As in the previous design, sealing of the tubes to the header was accomplished by compression of the grommets between the tubes and the header. However, in this alternative design, the tubes were assembled to the headers individually thereby avoiding high assembly forces. This allowed the construction of very large radiators for heavy construction equipment. However, it has been found that the use of tubes with round ends limits this design to cores having rather wide tube spacing which results in relatively poor thermal performance compared to most radiator core designs.
U.S. Pat. Nos. 4,756,361 and 5,205,354 describe a radiator which utilizes tubes which are circular in cross-section throughout their length. This type of design is shown in FIG. 4 in which tubes 22 are pressed through collar openings 25 in flat plate fins 24. The tube ends extend through silicone rubber grommets 42 which are disposed in openings within header plate 28. The grommets have a central peripheral groove and top and bottom lips or flanges which extend outward on the top and bottom of the header plate. Because of its round tubes, this design also suffers from poor thermal performance compared to most radiator designs and must have close tolerances to achieve the required compression of the grommet between the tube and header opening to seal the joint. U.S. Pat. Nos. 5,052,475 and 5,226,235 disclose use of circular grommets to seal circular tubes into soldered tanks and welded tanks, respectively. British Patent No. 29,777 discloses the use of round tubes and grommets with a tube plate cast integrally with the header, but the tube openings are drilled or otherwise formed in the tube plate after the casting is made.
Currently, air-to-air heat exchangers using brazed aluminum cores having oval tubes are being produced commercially. Aluminum headers having oversized oval openings are welded to cast aluminum manifolds. (Full cast tank/header combinations are not believed to be used.) Oval silicone rubber grommets, otherwise similar to those described in the aforementioned '361 patent, are inserted into the openings in the headers of the welded tanks. Such extensive soldering, welding and machining relating to the tank/header combinations heat exchangers utilizing grommets adds undesirable handling and other costs to the manufacturing process.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved heat exchanger which utilizes grommets in a tube-to-header joints and may be readily manufactured without welding or soldering.
It is a further object of the present invention to provide an improved method of assembly of a heat exchanger which utilizes grommets in the tube-to-header joint of a tank which integrally includes the header manufactured with a minimum of machining.
It is a further object of the present invention to provide an improved method of manufacturing a header and tank assembly for a heat exchanger.